This invention relates to light transmissive panels such as windows or windshields and, more particularly, to a method and apparatus for limiting light transmission through such panels as a function of light incidence angle.
There are numerous applications in which it is desirable to allow light to pass through a light transmissive panel without significant attenuation in a range of incidence angles while greatly attenuating light impinging on the panel at other angles. For example, in automobile windshields, it is generally desirable not to affect a driver's view by attenuating light transmission through the windshield in a substantially horizontal, forward direction. At the same time, it may be desirable to block or significantly attenuate bright, overhead sunlight through the windshield as well as through other windows of the vehicle since such sunlight tends to heat the automobile and to fade or otherwise damage cloth, vinyl, or leather interiors. As another example, it may be desirable to attenuate sunlight shining through windows of a building in order to reduce its heating effect while maintaining the ability of an occupant to see through the window. Still another use may be in directional control of incident light for use with optical signalling or in exiting light from auto headlamps or traffic signals.
It has been known to provide light attenuation through windows, windshields, and other light transmissive panels by use of polarized film attached to or held adjacent such panels. For example, U.S. Pat. No. 4,123,141 describes a window system incorporating polarized panels which may be adjusted between a pair of positions to either minimize or maximize light transmission therethrough. Similarly, U.S. Pat. No. 4,090,732 describes an attachment to an automobile visor comprising a pair of polarized panels, one of which is polarized at a different angle with respect to the other so that when used in combination, glare is substantially reduced below that normally obtained by use of only one panel.
While the above described systems are effective to minimize light transmission, these systems are not generally incidence angle sensitive. Their primary mode of operation is to reduce light transmission at all angles. For application to automobile windshields, systems such as that in U.S. Pat. No. 4,090,732 are designed to cover only limited areas since coverage of the entire windshield would impede the vehicle operator's vision. Thus, it would be desirable to provide a system for reducing light transmission from overhead sunlight with minimal effect on an operator's forward vision.